Method of transmission of a video sequence of images that have to be color transformed using LUT

ABSTRACT

Before the transmission of the source color values of the images of each group G k  of images of this sequence, a partial color transform LUT related to this group G k  is generated, then transmitted, —and, a resultant color transform LUT related to said group G k  is reconstructed by combining the transmitted partial color transform look up table of this group G k  with, if any, all the other partial color transform look up tables that have been previously transmitted, —then each images of said group are color transformed using this LUT. The invention allows a good precision of color transformation with a limited bandwidth.

This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/EP2008/057903, filed Jun. 20, 2008, which was published in accordance with PCT Article 21(2) on Dec. 31, 2008 in English and which claims the benefit of European patent application No. 07301165.2, filed Jun. 27, 2007.

BACKGROUND

1. Field of the Invention

The invention relates to a method of transmission of at least one video sequence of images that is divided into a plurality of groups of successive images,

comprising, for each group of successive images, the following steps:

-   -   transmitting source color values that represent image(s) of said         group in a source color space from an emitter towards a         receiver,     -   then transforming transmitted source color values into         destination color values that represent image(s) of said group         into a destination color space, by using a so-called resultant         color transform look up table that is related to said group.

The transmission of the color transform look up table that is necessary to transform source color values into destination color values is a main issue that is addressed by the invention.

2. Discussion of Related Art

The invention relates generally to color management and in particular to color transform of color values of images that are transmitted through a video channel (as HDMI compliant wire link), when such images are grouped into at least one video sequence. When the images move along the post-production workflow from a so-called source color device to another so-called destination color device, it is necessary to transform the colors of a specific image of a video sequence from the color space associated with the source color device to the color space associated with the destination color device. Color device can be a display device as a copying device. Color device can be virtual, and then be associated for example with the well-known sRGB color space. The above-mentioned transformation of color values is usually non-linear. In practice, this transformation is done using predefined coloured tables, so-called Look-Up-Tables (LUTs). A LUT contains for each entry color C_(IN) an output color C_(OUT). Being a table, a LUT is inherently discrete in color space. Often the LUT entries are quantized coarser than the color values in order to limit its size. LUTs are often applied to different color values of images by using dedicated hardware based on specifically designed memory structures. Such hardware loads a LUT during initialization and then starts to apply the LUT to the color values of images. But, the LUT may change in time, from one sequence to a following one, for example to adapt to the temporal changing image content or to adapt to the temporal changing viewing conditions such as ambient light. In this case, for a new sequence of images, a new LUT has to be loaded into the specific hardware. The color display may be at a distant place and images of a sequence are transmitted one after the other. If the LUT needs to be updated from a specific sequence on, images of this sequence have to be transmitted such that the whole LUT is received in time for the starting of the mentioned sequence. Since transmission bandwidth of the video channel is usually limited, at least one of the following parameters is limited: the frequency of LUT update, the number of LUT entries, and the precision of LUT output values. It has to be pointed out that limited memory on receiver side has a similar effect as that of limited bandwidth.

In the document U.S. Pat. No. 5,923,316, a color transformation is disclosed, that uses a look-up-table the size of which may dynamically vary to accommodate varying parameters, such as available memory. Reducing the size of the lookup table will increase the error component in the color transformation, which reduces picture quality. However a full speed video playback can be achieved by truncating least significant bits of the LUT, until the truncated lookup table fits the memory space. A drawback of such a transmission is that the LUT are finally not entirely but only partially transmitted, because of the truncation. In the document U.S. Pat. No. 5,736,989, a method is disclosed for reducing computation time and memory space required to generate and store color conversion interpolation tables (=LUT). A partially filled, regular LUT that is filled on an “as needed” basis is disclosed. Fill status is represented by status bits. The problem of the transmission of entire large LUTs is not solved, since required LUT entries need to be calculated before an incoming color value can be transformed.

The document U.S. Pat. No. 6,771,275 discloses using a look-up table that includes both sampled data entries and difference values (also known as “differentials”). The difference values correspond to the rates of change from each table entry and its neighbors in the table. Then, the whole LUT is approximated using a sub-LUT-0, a sub-LUT-delta and an interpolation engine. Although this method resolves the problem of how to get a large sized LUT by transmitting an approximated LUT and by using an interpolation engine, a drawback of this method is that the whole LUT is only an approximation.

The document U.S. Pat. No. 6,621,498 discloses the tiling of a LUT that is used for color transform. As color values (“RGB parameters”) are typically represented by three 8-bit values (for each of the R, G, and B components), the size of look up tables are prohibitively expensive and physically impractical. Therefore, such look up tables are typically partitioned into different “tile”, each tile being for instance represented by three 4-bit values. Output values that are not directly represented by nodes of these tiles are determined by a form of interpolation among nodes of the look up table (LUT). Tiling look up tables can be even and regular, i.e. each tile is a cube, and the dimensions of each cube are the same. To resolve the problem of how to store (or to transmit) a large sized LUT, it is proposed to tile non-regularly look up tables. This document propose to tile a LUT based on multi-dimensional, non separable, non-regular sampling of the basic LUT into sub-LUTs, in order to increase precision where required and low memory footprint at same time. Local sampling granularity is restricted to be one of a number of discrete granularity grains in between a minimal and a maximal grain. Hereby a sliced or tiled LUT organization results. This method is complicated to implement. Moreover, the division of the basic LUT is made according to the degree of complexity of the relationship between zones of the source color space and zones of the destination color space, whatever the actual needs of transformation, i.e. the color content of the images to transform. According to the document U.S. Pat. No. 5,652,831, “An input image data is divided into upper bits and lower bits . . . . The upper bits are used by a three-dimensional color correction look-up table for outputting correct color reference points. The lower bits are used by a three-dimensional interpolation coefficient generating table for outputting the coefficients corresponding to each reference point. The color correction values of inputted image data can be conclusively computed in accordance with the two sets of data (i.e. two LUTs), i.e. the upper bits and the lower bits . . . . Then, the appearance equivalent color characteristics are analyzed according to the inputted image data to determine the dividing ratio of the upper bits and the lower bits (i.e. the division between the two LUTs), wherein the computation is scalable because the ratio of the numbers of the upper bits and the lower bits can be adjusted. The method may skip unnecessary interpolation computation and choose the color patch directly so that the computation accuracy and speed are increased. Moreover, the secondary error caused by the distortion of interpolation computation can also be avoided. Preferably a fuzzy variable point interpolating method may be used to quickly quantize the color coordinate and determines the local attribute in color space for the data characteristic of an input image.” One drawback of methods that are disclosed in this document is that the division of the LUT is made according to the bit depth, whatever the actual needs of transformation, i.e. the color content of the images to transform.

Other documents deals with the transmission of color palettes as U.S. Pat. No. 7,091,985.

SUMMARY

An object of the invention is to avoid the aforementioned drawbacks.

For this purpose, the subject of the invention is a method of transmission of at least one video sequence of images that is divided into a plurality of groups G₁, G₂, . . . , G_(k), . . . , G_(V) of successive images,

comprising, for each group G_(k) of successive images, the following steps:

-   -   transmitting source color values that represent image(s) of said         group G_(k) in a source color space,     -   then applying a color transform adapted to transform transmitted         source color values into destination color values that represent         image(s) of said group G_(k) into a destination color space, by         using a resultant color transform look up table that is related         to said group G_(k),

wherein, for said color transformation, before the transmission of the source color values of the images of said group G_(k),

-   -   a partial color transform look up table {(R_(h), G_(h), B_(h),         R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(Gk)]} related to said group         G_(k) is generated from a global color transform look up table         representing said color transform, the size Ñ_(Gk) of said         partial color transform look up table being smaller than the         size T³ of said global color transform look up table,     -   then said generated partial color transform look up table of         said group G_(k) is transmitted,     -   and, said resultant color transform look up table related to         said group G_(k) is reconstructed by combining the transmitted         partial color transform look up table of this group G_(k) with,         if any, all the other partial color transform look up tables         that have been previously transmitted and that are related to         other groups of successive images.

Preferably, the generation of the different partial color transform look up tables is performed such that none of these different partial color transform look up tables overlaps with another one.

Source color values that represent an image may be only part of the whole color values of this image; the image transmission may be partial, scalable, in lower resolution, i.e. not all color values are to be transmitted.

The global color transform look up table is able to transform any source color value within the source color space into a destination color value within the destination color space.

The size of a color transform look up table corresponds to the numbers of pairs of input-output color values that are included within this color transform look up table.

Each of said look up tables comprise generally a plurality of pairs of color values, each pair comprising one source color value as an input and the corresponding destination color value as an output; as a variant where, for instance, source color values of a look up table are regularly sampled on a grid, each of said look up table comprise a plurality of pairs, each pair comprising the position of one source color value on said grid (and the reference of that grid, if necessary) as an input and, as previously, the corresponding destination color value as an output.

To transform source color values that are substantially different from all source color values (as an input) of the resultant color transform table that is used to transform them, interpolation is performed from the closest source color values (as an input) of this resultant color transform table; well known usual interpolation algorithms are used.

Generally, the resultant color transform look up tables are reconstructed not only by combining the transmitted partial color transform look up table, but also, for instance, by applying interpolation functions to generate missing input-output pairs of color values that are necessary to perform the color transform.

Preferably, the resultant color transform look up table are regularly sampled by using either color values of partial color transform look up tables or interpolated color values that are interpolated from theses color values of partial color transform look up tables.

Preferably, color values being digitized (using for instance D digits or bits), the transmission of color values is performed without truncation of their digits. When transmitting any partial color transform look up table, all digits of each color value of this table are then transmitted. Advantageously, source color values that are close to source color values that are part (as an input) of the resultant color transform table that is used to transform them are indeed very precisely transformed.

From group to group of images, the resultant color transform look up table incorporates more and more partial color transform look up tables, then being closer and closer to the global color transform table, as far as the transmission of the images of the video sequence is going on. For short video sequences, the transmission may end before the resultant color transform look up table is close enough to the global color transform table. For longer video sequences, as far as the resultant color transform look up table is considered as close enough to the global color transform table, no other partial color transform look up tables are generated and transmitted and no update of the resultant color transform look up table takes place, and the last group G_(V) of images comprises all the remaining images to transmit.

According to the invention, the resultant color transform look up table that is used to calculate the destination color values is updated by reconstruction any time a partial color transform look up table is transmitted from the emitter towards the receiver. Images of the video sequence that are transmitted after a resultant color transform look up table is updated are transformed by using this updated resultant color transform look up table, until a next partial color transform look up table is transmitted. Images of the video sequence that are transmitted between two successive transmissions of color transform look up tables then form a group of images that are all transformed using the same resultant color transform look up table. Such a group may contain only one image. From group to group of images of the same video sequence, the resultant color transform look up table incorporates more and more partial color transform look up tables, then being closer and closer to the global color transform, then incorporating more and more input source color values of this global color transform. According notably to the duration of the video sequence and to the size of the global color transform look up table representing the color transform, the global color transform look up table is entirely transmitted or not entirely transmitted.

Thank to the invention, a good comprise is obtained between the precise methods of transmission of the prior art which requires the transmission of the entire global color transform look up table before starting to transmit the images to transform and the imprecise methods of transmission of the prior art in which only one partial color transform look up table or only truncated bits of the global color transform look up table are transmitted. According to the invention and contrary to what is disclosed in the document JP11/041,625, because there are a plurality of groups G_(k) of successive images in the video sequence, i.e. at least two groups, the color transform is applied to the images of at least the second group by using a resultant color transform look up table that is reconstructed by combining at least two partial color transform look up tables, i.e. the partial color transform look up table related to the first group and the partial color transform look up table related to the second group.

Thank to the invention, the precision for the color transformation is increasing as far as the images are transmitted, without requiring large bandwidth for the transmission of the color transform look up tables.

At least one channel is used for transmission, which can be part of a wire or a wireless link; HDMI-compliant cable can be used for transmission, as any usual data-bus comprising several TMDS (Transition Minimized Differential Signaling) video data channels, a DDC (Display Data Channel) channel and an optional CEC (Consumer Electronics Control) channel. Preferably, the channel that is used for the transmission of the partial color transform look up tables is different from the channel that is used for the transmission of images, i.e. for the transmission of source color values related to these images. When using a HDMI-compliant cable, the DDC or the CEC channel is preferably used for the transmission of the partial color transform look up tables.

Generally, transmission is performed from an emitter towards a receiver. The emitter comprises an image emission control unit and a look up table emission control unit that are adapted to control the transmission of color values (representing the images or being part of the look up tables) towards the receiver. The receiver comprises an image reception control unit and a look up table reception control unit that are adapted to control the transmission of color values (representing the images or being part of the look up tables) from the receiver. Preferably, the receiver is connected to a color device, as a display device or a copy device.

On the emitter side, source color values of images are provided in a source color space. The source color space can be device dependant or device-independent. The source color space can be dependant on a virtual device, as, for example, the usual sRBG color space. Preferably, on the receiver side, destination color values are sent to the color device to which the receiver is connected, and the destination color space is dependant on this color device. According to a variant, source and/or destination color space can be device-independent, as for example the XYZCIE 1931 visual color space, or the LabCIE perceptually uniform visual color space.

Preferably, the generated partial color transform look up table of said group G_(k) is transmitted though a color transform channel, and the number Ñ_(Gk) of input-output pairs within this partial color transform look up table of said group G_(k), i.e. the size Ñ_(Gk) of this partial color transform look up table, is proportional to the bandwidth of said color transform channel.

Preferably, if said color values are transmitted through an image channel having an image bandwidth BWIM and if generated partial color transform look up table of said group G_(k) is transmitted though a color transform channel having a color transform bandwidth BWCT_(Gk), then the number Ñ_(Gk) of input-output pairs within said partial color transform look up table, i.e. the size Ñ_(Gk) of this partial color transform look up table, is inferior or equal to: Q×M_(G(k−1))/(1+log₂T/D)×BWCT_(Gk)/BWIM, where Q is the number of pixels within each transmitted image, M_(G(k−1)) is the number of images in the previous group G_(k−1), if any, of transmitted image, and T is defined such that T³ is the size of the global color transform look up table.

Then, the definition of the number M_(G(k−1)) of images within each group G_(k) and the definition of the number Ñ_(Gk) (or Ñ_(G)) of input-output pairs within each partial color transform LUT is the basis for the definition of each group G_(k), taking into account the available bandwidth. Generally, the number M_(G(k−1)) of images within each group G_(k−1) is constant and equal to M_(G); then, for each group G_(k), Ñ_(Gk)≦Q×M_(G)/(1+log₂T/D)×BWCT_(Gk)/BWIM.

Preferably, during the transmission of the images of a group G_(k) through an image channel, partial color transform LUT of a following group G_((k+1)) is transmitted through a color transform channel, in order to prepare in advance the reconstruction of the resultant color transform look up table that is related to said following group G_((k+1)).

From one video sequence to another one, actual color transform may change for various reasons, that may depends on the video content, as far as, for instance, this sequence contains night scenes, indoor scenes and daylight outdoor scene having then substantially different colors and colors distribution. Preferably, to adapt the color transmission to the actual requirements of the images of a group which has to be transmitted, before the transmission of the source color values of the images of said group G_(k):

-   -   N_(Gk) more frequent source color values {(R_(g), G_(g),         B_(g))\gε[1, N_(Gk)]} of said group G_(k) are determined,     -   then, to generate the partial color transform look up table         {(R_(h), G_(h), B_(h), R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(Gk)]}         related to said group G_(k), for each (R_(g), G_(g), B_(g)) of         these frequent source color values {(R_(g), G_(g), B_(g))\gε[1,         N_(Gk)]}, a source color value or input (R_(h), G_(h), B_(h)) is         extracted with its associated output (R′_(h), G′_(h), B′_(h))         from the source color values or inputs of the global color         transform look up table which are the closest to said source         color value (R_(g), G_(g), B_(g)) in the source color space.

When the same pair is extracted in connection with two different frequent source color values, then Ñ_(Gk) is inferior to N_(Gk). The number N_(Gk) of successive images into each group G_(k) is then the basis for the definition of each group of images. As previously explained, this definition is preferably based also on bandwidth considerations.

Generally the distribution of source color values of the images of each group are statistically analyzed to determine the more frequent source color values of this group. While choosing the more frequent source colors, the measure of frequency may be weighted by a color importance term that is higher for important colors and lower for less important colors. Important colors can either be important colors in an artistic sense, or colors that can be displayed with higher precision on a display device or colors that are perceived by the human eye with a higher precision.

Preferably, the (T³) input-output pairs of the global color transform look up table are distributed into N grids L₁, . . . , L_(n), . . . , L_(N) having increasing numbers X_(L1)< . . . <X_(Ln)< . . . <X_(LN) of input-output pairs from a highest grid L₁ down to a lowest grid L_(N) such that any source color value of the input-output pairs of a lower grid L_(n) is located, in the source color space, between adjacent source color values of its next higher grid L_(n−1); then, in order of decreasing frequency of the more frequent source color values {(R_(g), G_(g), B_(g))\gε[1, N_(Gk)]} of said group G_(k), for each (R_(g), G_(g), B_(g)) of these more frequent source color values, a source color value (R_(h), G_(h), B_(h)) or input which has not yet been previously extracted is extracted with its associated output (R′_(h), G′_(h), B′_(h)) from the highest grid L_(n) of the global color transform look up table, starting from the highest grid L₁ down to the lowest grid L_(N).

In a 3D color space, any source color value of a given grid is generally located at the center of a segment delimited by 2¹ adjacent inputs or source color values of its next higher grid, or a the center of a square delimited by 2² adjacent inputs or source color values of its next higher grid, or a the center of a cube delimited by 2³ adjacent inputs or source color values of its next higher grid. Instead of being located at the center of this segment, square, or cube, this source color value can be located elsewhere inside this segment, square, or cube, without departing from the invention. Preferably, in any given grid L_(n), there is only on source color value inside any segment, square, or cube delimited by adjacent source color values of its next higher grid L_(n−1).

As the generation of the partial color transform LUTs is done from upper coarser grids down to lower finer grids of the global color transform look up table, even colors values that are far away, in the source color space, from the most frequent color values, will be correctly transformed by the partial color transform LUT.

Preferably, instead of transmitting a source color value or input within each pair of the partial color transform look up table, only the position of the source color values within its grid is transmitted with the reference n of the grid; then each input-output pair contains this position with this reference n as the input and the corresponding destination color value as previously; one advantage of this variant is that the number of digits that are used to transmit each pair of the partial look up tables is lowered, then limiting the bandwidth requirements. Preferably, in each grid Ln, the inputs of the input-output pairs are regularly distributed in the source color space.

The subject of the invention is also a method of displaying at least one video sequence of images in a destination color space, wherein that video sequence of images is transmitted by using a method of transmission according the invention. The destination color space is associated with the color device that is used to display the images; such a color device can be an actual color device, as a LCD or plasma panel or a projector, or a virtual device as a standard for broadcasting video images, as, for instance, the so-called sRGB color space. The subject of the invention is also a system of transmission of at least one video sequence of images that it is able to implement a method of transmission according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood on reading the description which follows, given by way of non-limiting example and with reference to the appended figures in which:

FIG. 1 is a schematic diagram of a system of transmission according to an embodiment of the invention;

FIG. 2 shows, in the source color space, a distribution of the inputs of the input-output pairs of the global color transform LUT according to a variant of an embodiment of the transmission method according to the invention.

DETAILED DESCRIPTION

In reference to FIG. 1, the transmission system according to an embodiment of the invention comprises an emitter 1, a receiver 2 and a transmission link 3 between the emitter 1 and the receiver 2.

The emitter 1 comprises an image emission control unit 12 and a color transform LUT emission control unit 16. The receiver 2 comprises an image reception control unit 22 and a color transform LUT reception control unit 24. The transmission link 3 comprises an image channel 31 that is adapted to transmit the image data that are sent by the image emission control unit 12 to the image reception control unit 22, and a color transform LUT channel 32 that is adapted to transmit the color transform LUT data that are sent by the color transform LUT emission control unit 16 to the color transform LUT reception control unit 24.

The emitter 1 comprises also an image data storing unit 11 that is adapted to deliver source color values of successive images of a video sequence to the image emission control unit 12. The emitter 1 comprises also a color transform data storing unit 13, a partial color transform LUT generating unit 14, and a histogram generating unit 15. The color transform data storing unit 13 is adapted to deliver a global color transform LUT that is related to a video sequence that the image data storing unit 11 is going to deliver to the image emission control unit 12. The histogram generating unit 15 is adapted to generate histograms of source color values of successive images that are delivered to the image emission control unit 12. The partial color transform LUT generating unit 14 is adapted to generate, from the global color transform LUT delivered by the color transform data storing unit 13, a partial color transform LUT that is related to the most frequent source color values that have been identified on the histogram of source color values that is provided by the histogram generating unit 15, and to deliver this partial color transform LUT to the color transform LUT emission control unit 16.

The receiver 2 comprises also a color device 21. A destination color space is attached to this color device, the characteristics of which are stored in the color transform data storing unit 13, in order to enable it to deliver a global color transform LUT that is adapted to transform the source color values of images of the video sequence into destination color values. The receiver 2 comprises also a color transform unit 23 that is adapted to transform the source color values that have been received by the image reception unit 22 into destination color values and to deliver them to the color device 21, in order to display or to copy the corresponding images of the video sequence. The global color transform LUT is adapted in a manner known per se to obtain a good reproduction of the images with this color device 21, and may include gamut mapping algorithm if necessary. The receiver 2 comprises also a resultant color transform LUT generating unit 25 that is adapted to reconstruct a resultant color transform LUT from the successive partial color transform LUT that are delivered by the color transform LUT reception unit 24, and to deliver the resultant color transform LUT to the color transform unit 23 to allow it to transform the source color values.

The color device 21 can be for instance a display device or a copy device. More generally, color device also include varying media such as paper for color prints, film for film projection or digital media (for example DVD) for monitor display. In this sense, a color device may include several physical devices such as a DVD recorder and a connected monitor.

A transmission method according to an embodiment of the invention will now be described. Prior to the application of the transmission method:

-   -   source color values of successive images Im₁ to Im_(V) of the         video sequence S_(V) are stored as digitized numbers in the         image data storing unit 11; any source color value of any pixel         Pix_(i) of a given image Im_(q) of this sequence S_(V) is a         triplet R_(iq), G_(iq), B_(iq) of numerical values that are         coded with a bit depth D=10 and that are then comprised between         0 and 1023 (0 and 1023 included). Each image comprises Q pixels         Pix_(i), then 0≦i<Q.     -   color transform data are stored in the color transform data         storing unit 13, that are able to transform source color values         of the video sequence S_(V) that are expressed in a source color         space into destination color values of the same video sequence         S_(V) that are expressed in the destination color space that is         related to the color device 21. Such color transform data         globally represent a given color transform f(•) that allows to         calculate a global color transform LUT where output destination         color values R′_(j), G′_(j), B′_(j) are calculated from input         source color values R_(j), G_(j), B_(j) as follows: (R′_(j),         G′_(j), B′_(j))=f(R_(j), G_(j), B_(j)). The global color         transform LUT is then represented as {(R_(j), G_(j), B_(j),         R′_(j), G′_(j), B′_(j))\jε[0, T³−1]}, with T being the         resolution of this global color transform LUT, e.g. T=257=2⁸+1,         resulting in a number of T³ LUT values. Here, the LUT source         color entries are chosen on a regular RGB grid as follows:

${\left( {R_{j},G_{j},B_{j}} \right) = \left( {\frac{p\left( {2^{D} - 1} \right)}{T - 1},\frac{q\left( {2^{D} - 1} \right)}{T - 1},\frac{j\left( {2^{D} - 1} \right)}{T - 1}} \right)},$ with p=j mod T²; q=j mod T; 0≦j<T³, where <<mod>> is the abbreviation of <<modulo>>, and where D, already defined, is the number of bits of image signals, i.e. of LUT inputs and outputs.

Such a color transform as defined above with its global color transform LUT may include gamut mapping algorithms that are adapted for the specific transformation from the source color space towards the destination color space or color transformations based on gamut boundary descriptors of the source color gamut and of the destination color gamut in a common color space. While the invention relates to any type of color transform, the preferred embodiment is a solution for a colour transform from a device-dependant RGB (red, green, blue) color space into another device-dependant R′G′B′ color space. Other color spaces such as YCbCr or device-independent color space as CIE XYZ or CIE L*a*b* can be used. Moreover, color space coordinates as RGB, YCbCr, CIE XYZ or CIE L*a*b* may be encoded according to standards such as IEC 61966-2-4 or SMPTE S274M. Furthermore, the preferred embodiment is a solution for so-called 3D Look-Up Table (LUT). Other color transforms such as several one-dimensional LUTs, linear matrices, or any mathematical transforms can be used instead of the global color transform LUT. As a variant, instead of being calculated as described above, color transform may be defined directly as a global color transform LUT and stored directly as is in the color transform data storing unit 13.

The transmission method according to the embodiment of the invention then comprises the following steps:

-   1) The image data storing unit 11 then provides the image emission     control unit 12 and the histogram generating unit 15 with a first     group G₁ of successive images of the video sequence S_(V), by     delivering to these units the corresponding source color values. -   2) the histogram generating unit 15 analyses statistically the     source color values that are provided by the image date storing unit     11 and output the N_(G1) most frequent source color values {(R_(g),     G_(g), B_(g))\gε[1, N_(G1)]} of the images of the first group G₁. -   3) for each (R_(g), G_(g), B_(g)) of these most frequent source     color values {(R_(g), G_(g), B_(g))\gε[1, N_(G1)]}, the partial     color transform LUT generating unit 14 extracts, from the T³ source     color values or inputs of the global color transform LUT, a source     color value or input (R_(h), G_(h), B_(h)) which is the closest to     said source color value (R_(g), G_(g), B_(g)) in the source color     space, and combine this extracted input (R_(h), G_(h), B_(h)) with     its associated output (R′_(h), G′_(h), B′_(h)) to form a pair     (R_(h), G_(h), B_(h), R′_(h), G′_(h), B′_(h)) of input-output; all     extracted pairs then generate a partial color transform look up     table {(R_(h), G_(h), B_(h), R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(G1)]}     that is related to the group G₁ and that is then delivered to the     color transform LUT emission control unit 16. The number     Ñ_(G1)≦N_(G1) of colors is smaller than the number T³ of LUT entries     and is chosen with respect to the available transmission bandwidth     as explained forward. -   4) the color transform LUT emission control unit 16 then transmits     the input values and all digits of the output values of this partial     color transform LUT (when the input values lie on a grid, in     practice full RGB values will not be transmitted but grid     coordinates): {(R_(h), G_(h), B_(h), R′_(h), G′_(h), B′_(h))\hε[1,     Ñ_(G1)]} to the color transform LUT reception control unit 24     through the color transform channel 32 of the transmission link 3. -   5) the resultant color transform LUT generating unit 25 which is     provided with the partial color transform LUT: {(R_(h), G_(h),     B_(h), R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(G1)]}, stores this partial     color transform LUT as the resultant color transform LUT of the     1^(st) group G₁ of successive images, applying preferably but     optionally an interpolation function to generate the missing colors     in the resultant color transform LUT, and delivers this resultant     color transform LUT to the color transform unit 23. -   6) the image emission control unit 12 then transmits the source     color values of the successive images of the first group G₁ to the     image reception control unit 22 through the image channel 31 of the     transmission link 3. -   7) then, the color transform unit 23 applies the resultant color     transform LUT to the source color values of the images of the first     group G₁, in order to transform in a manner known per se all these     source color values into destination color values. -   8) the color device 21, when being for instance a display device,     can then display the successive images of the first group G₁ of the     video sequence V_(S) by using the destination color values that are     provided by the color transform unit 23. -   9) The image data storing unit 11 then provides again the image     emission control unit 12 and the histogram generating unit 15 with a     second group G₂ of successive images of the video sequence S_(V), by     delivering to these units the corresponding source color values. -   10) the histogram generating unit 15 analyses statistically these     source color values and, after having excluded the N_(G1) most     frequent source color values {(R_(g), G_(g), B_(g))\gε[1, Ñ_(G2)]}     of the previous group G₁, output the other N_(G2) most frequent     source color values {(R_(g), G_(g), B_(g))\gε[1, Ñ_(G2)]} of the     second group G₂ with Ñ_(G2)≦N_(G2) and Ñ_(G2)≦T³−Ñ_(G1). -   11) for each (R_(g), G_(g), B_(g)) of these most frequent source     color values {(R_(g), G_(g), B_(g))\gε[1, Ñ_(G2)]} of the second     group G₂, the partial color transform LUT generating unit 14     extracts, from the T³ source color values or inputs of the global     color transform look up table, a source color value or input (R_(h),     G_(h), B_(h)) which is the closest to said source color value     (R_(g), G_(g), B_(g)) in the source color space and which has not     yet been extracted for the generation of the previous partial color     transform look up table at the step 3) above, and combine this     extracted input (R_(h), G_(h), B_(h)) with its associated output     (R′_(h), G′_(h), B′_(h)) to form a pair (R_(h), G_(h), B_(h),     R′_(h), G′_(h), B′_(h)) of input-output; all extracted pairs then     generate a partial color transform look up table {(R_(h), G_(h),     B_(h), R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(G2)]} that is related to the     group G₂ and that is then delivered to the color transform LUT     emission control unit 16. The number Ñ_(G2)≦N_(G2) of colors is     smaller than the number T³−Ñ_(G1) of remaining LUT entries and is     chosen with respect to the available transmission bandwidth as     explained forward. -   12) preferably during the above step 6 of transmission of the source     color values of the successive images of the previous group G₁, the     color transform LUT emission control unit 16 transmits all digits of     at least the output values this partial color transform LUT {(R_(h),     G_(h), B_(h), R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(G2)]} to the color     transform LUT reception control unit 24 through the color transform     channel 32 of the transmission link 3. -   13) the resultant color transform LUT generating unit 25 then     combines this partial color transform LUT {(R_(h), G_(h), B_(h),     R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(G2)]} with the partial color     transform LUT {(R_(h), G_(h), B_(h), R′_(h), G′_(h), B′_(h))\hε[1,     Ñ_(G1)]} of the previous group, to reconstruct a new resultant color     transform LUT for this second group G₂, applying preferably but     optionally an interpolation function to generate the missing colors     in the resultant color transform LUT, and delivers this new     resultant color transform LUT to the color transform unit 23. -   14) the image emission control unit 12 then transmits the source     color values of the successive images of the second group G₂ to the     image reception control unit 22 through the image channel 31 of the     transmission link 3. -   15) then, the color transform unit 23 applies the new resultant     color transform LUT to the source color values of the second group     G₂, transforming all these source color values into destination     color values. -   16) the color device 21, when being or instance a display device,     can then display the successive images of the second group G₂ of the     video sequence V_(S) by using the destination color values that are     provided by the color transform unit 23.

The above 1) to 8), then 9) to 16) steps are repeated for each of the following groups G₃, . . . , G_(k), . . . G_(V) of successive images of the video sequence S_(V):

-   -   the histogram generating unit 15 analyses statistically the         source color values of the images of a group G_(k), and, after         having excluded the most frequent source color values of the         previous groups G₁ to G_(k−1), output the other N_(Gk) most         frequent source color values {(R_(g), G_(g), B_(g))\gε[1,         N_(Gk)]} of the group G_(k).     -   the resultant color transform LUT generating unit 25 then         combines the partial color transform LUT {(R_(g), G_(g), B_(g),         R′_(g), G′_(g), B′_(g))\gε[1, Ñ_(Gk)]} that has been again         generated as in steps 3) and 11) above by the partial color         transform generating unit 14 with all the partial color         transform LUT of the previous groups G₁ to G_(k−1), to         reconstruct a new resultant color transform LUT for this group         G_(k), applying preferably but optionally an interpolation         function to generate the missing colors, if any, in the         resultant color transform LUT, and delivers this new resultant         color transform LUT to the color transform unit 23.     -   then, the color transform unit 23 applies the new resultant         color transform LUT to the source color values of the group         G_(k), transforming all these source color values into         destination color values.

As, for each group G_(k) of successive images of the video sequence S_(V), the histogram generating unit 15 excludes the most frequent source color values of the previous groups G₁ to G_(k−1) (see for instance step 10 above), and as for each (R_(g), G_(g), B_(g)) of these most frequent source color values {(R_(g), G_(g), B_(g))\gε[1, N_(Gk)]} of this group G_(k), the partial color transform LUT generating unit 14 extracts only source color values or inputs (R_(h), G_(h), B_(h)) which have not yet been extracted for the generation of the previous partial color transform look up table (see for instance step 11 above), the generation of the different partial color transform look up tables is performed such that none of these different partial color transform look up tables overlaps with another one. If the histogram generating unit 15, after having analysed statistically the source color values of the images of one group G_(q) and after having excluded the most frequent source color values of the previous groups G₁ to G_(q−1), output zero most frequent source color values (N_(Gq)=0), because, for instance, the source color values of the images of this group G_(q) are all identical or similar with the source color values of the images of another previous group of the video sequence, then the partial color transform LUT generating unit 14 would for instance extract, from the T³ source color values or inputs of the global color transform look up table, an arbitrary selection of source color values or inputs (R_(h), G_(h), B_(h)) which have not yet been extracted for the generation of the previous partial color transform look up tables. The size Ñ_(Gq) of this “arbitrary generated” partial color transform LUT would also be inferior to T₃ as for the other partial color transform LUTs.

As soon as the total number Ñ_(G1)+Ñ_(G2)+ . . . +Ñ_(GV) of input-output pairs of the different partial color transform look up tables, including a last partial color transform look up table under generation associated with a group G_(V), reaches the total number T³ of input-output pairs of the global color transform look up table, this last group G_(V) will gather all the remaining images of the video sequence to transmit. After transmission of this last partial look up table, the resultant color transform LUT of this last group corresponds to the entire global color transform LUT which is then entirely and progressively transmitted. All images of the last group G_(V) are then color transformed using the maximum of precision, because the entire global color transform LUT can now be used for this transformation.

On the opposite, if the total number Ñ_(G1)+Ñ_(G2)+ . . . +Ñ_(GV) of input-output pairs of the different partial color transform look up tables, including a last partial color transform look up table under generation associated with a group G_(V′), does not reach the total number T³ of input-output pairs of the global color transform look up table before the transmission of the entire video sequence, then the transmission of images of the sequence S_(V) will stop before the transmission of the whole global color transform LUT and the images of the last group G_(V) will be transformed by using a resultant color transform LUT which is still only a part of the entire global color transform LUT. For this sequence, the global color transform LUT is then not entirely transmitted.

According to a variant, the transmission of the may also be stopped or paused at Ñ_(G1)+Ñ_(G2)+ . . . +Ñ_(Gk)<T³ according to a color distance criterion based on the distance of source color values and corresponding chosen closest source input color values of partial LUTs. As soon as the distance criterion is smaller than a predefined threshold, not partial LUT is transmitted.

Preferably, during the transmission of the Ñ_(Gk) pairs of the partial color transform LUT of a group G_(k) by using the color transform channel 32, the M_(G(k−1)) successive images Im₁, Im₂, Im_(q), . . . , Im_(mG(k−1)) of the previous group G_((k−1)) are transmitted through the image channel 31.

Let us assume that the available bandwidth of the image channel 31 is constant and equal to BWIM (generally expressed in Mbit/s); if the available bandwidth of the color transform channel 32 for the transmission of the partial color transform LUT of a group G_(k) is BWCT_(Gk) (expressed in the same units, generally in Mbit/s), then:

-   -   the time that is necessary to transmit this partial color         transform LUT of the group G_(k) will be at least equal to         (3×log₂T+3×D)×Ñ_(Gk)/BWCT_(Gk), if only the T positions of the         input values on the 3 axis of a grid are transmitted instead of         the corresponding actual source color values;     -   the time that is necessary to transmit the M_(G) images of the         group G_(k−1) will be equal to 3×D×M_(G(k−1))×Q/BWIM expected,         where Q, already defined, is the number of pixels within each         image.

It is expected that the time that is necessary to transmit this partial color transform LUT of the group G_(k) will be lower than the time that is necessary to transmit the M_(G) images of the group G_(k−1), i.e.: (3×log₂ T+3×D)×Ñ _(Gk) /BWCT _(Gk)≦3×D×M _(G(k−1)) ×Q/BWIM, or: (1+log₂ T/D)×Ñ _(Gk) ×BWIM≦Q×M _(G(k−1)) ×BWCT _(Gk), or: Ñ _(Gk) ≦Q×M _(G(k−1))/(1+log₂ T/D)×BWCT _(Gk) /BWIM.

Preferably, to optimize the available bandwidth, the size Ñ_(Gk) of the partial color transform LUT is adapted according to the equation: Ñ_(Gk)=Q×M_(G(k−1))/(1+log₂T/D)×BWCT_(Gk)/BWIM.

Generally, the available bandwidth for the partial color transform LUT is constant and equal to BWCT. Preferably, except for the last group G_(V), all groups G₁, G₂, . . . , G_(k), . . . , G_(V−1) comprise the same number of images M_(G).

The definition of the number M_(Gk) (or M_(G)) of images within each group G_(k) and the definition of the number Ñ_(Gk) (or Ñ_(G)) of inputs within each partial color transform LUT is considered as a definition of each group G_(k).

Thank to the invention, a good comprise is obtained between the precise methods of transmission of the prior art which requires the transmission of the entire global color transform look up table before starting to transmit the images to transform and the imprecise methods of transmission of the prior art in which only one partial color transform look up table or only truncated bits of the global color transform look up table are transmitted; thank to the invention, the precision for the color transformation is increasing as far as the images are transmitted, without requiring large bandwidth for the transmission of the color transform look up tables.

According to a variant of the steps 2) and 10) of formation of a partial color transform LUT of the method according to the invention, the T³ input-output pairs of the global color transform look up table are distributed into N layers or grids L₁, . . . , L_(n), . . . , L_(N) having increasing numbers X_(L1)< . . . <X_(Ln)< . . . <X_(LN) of input-output pairs from a highest layer L₁ down to a lowest layer L_(N). Each lower layer L_(n) is finer that its next upper layer L_(n−1), i.e. the definition of the layers or grids is higher and higher when the layers are lower and lower. Each lower grid L_(n) is intercalated into its next upper grid L_(n−1) such as any source color value of the input-output pairs of this lower layer L_(n) is located, in the source color space, between adjacent source color values of the input-output pairs of its next higher layer L_(n−1). In a 3D color space, adjacent source color values of a layer L_(n−1) may be a group of 2¹ closest source color values as two black dots along any of the main axis of the color space on FIG. 2, between which a white dot of a finer layer L_(n) is located as shown on FIG. 2; in such a 3D color space, adjacent source color values of a layer L_(n−1) may be also a group of 2² closest source color values as four black dots in any plane parallel to a main axis of the color space on FIG. 2, between which a white dot of a finer layer L_(n) is located as shown on FIG. 2, i.e. at the center of a square delimited by these four black dots; in such a 3D color space, adjacent source color values of a layer L_(n−1) may be at last a group of 2³ closest source color values as eight black dots forming a cube in the color space on FIG. 2, between which a white dot of a finer layer L_(n) is located as shown on FIG. 2, i.e. at the center of this cube.

According to this variant, the partial color transform LUT {(R_(h), G_(h), B_(h), R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(G1)], Ñ_(G1)=N_(G1)} which is related to any given group G_(k) of images is extracted as follows from this different layers or grids of the global color transform look up table:

-   -   in order of decreasing frequency, for each (R_(g), G_(g), B_(g))         of the most frequent source color values {(R_(g), G_(g),         B_(g))\gε[1, N_(Gk)]} of the images of the group G_(k), the         source color value or input (R_(h), G_(h), B_(h)) of the highest         layer L_(n) of the global color transform LUT which is the         closest to this most frequent source color values (R_(g), G_(g),         B_(g)) is extracted with its associated output (R′_(h), G′_(h),         B′_(h)), as far as this source color value or input has not yet         been extracted in association with another source color value,         for this group G_(k) or a previous more frequent one;     -   if the source color value or input of the highest layer L_(n)         which is the closest to this most frequent source color values         has already been extracted with its associated output in         association with another source color value, another closest         pair (R_(h), G_(h), B_(h); R′_(h), G′_(h), B′_(h)) of the global         color transform LUT is extracted from the same layer L_(n) or         from a lower layer L_(n+n′), where n′≧1.

Consequently, as for each (R_(g), G_(g), B_(g)) of the most frequent source color values of the images of the group G_(k) a pair (R_(h), G_(h), B_(h); R′_(h), G′_(h), B′_(h)) of the global color transform LUT is extracted, a partial color transform LUT {(R_(h), G_(h), B_(h), R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(Gk)], Ñ_(Gk)=N_(Gk)} is then generated.

Preferably, within each layer or grid L_(n), the source color value or input (R_(h), G_(h), B_(h)) are regularly sampled in the source color space.

An advantage of such a variant is that, as the generation of the partial color transform LUTs is done from upper coarser grids down to lower finer grids of the global color transform look up table, even colors values that are far away, in the source color space, from the most frequent color values, will be correctly transformed by the partial color transform LUT.

For this advantageous variant, any other association of the most frequent source color values {(R_(g), G_(g), B_(g))\gε[1, N_(G1)]} of the group G_(k) of images with pairs (R_(h), G_(h), B_(h); R′_(h), G′_(h), B′_(h)) of the global color transform LUT that is able to form a partial color transform LUT {(R_(h), G_(h), B_(h), R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(Gk)], Ñ_(Gk)=N_(Gk)} from different grids or layers ofl the global color transform look up table can be used without departing from the invention. The number Ñ_(Gk) of pairs in this partial color transform LUT can be inferior to the number of most frequent source color values N_(Gk) of the group G_(k), for instance when, if the source color value or input of the highest layer L_(n) which is the closest to this most frequent source color values has already been extracted with its associated output in association with another source color value, no other closest pair (R_(h), G_(h), B_(h); R′_(h), G′_(h), B′_(h)) of the global color transform LUT is extracted from the global color transform LUT.

In reference to FIG. 2, a more detailed example is now given of this advantageous variant.

The T³ input-output pairs of color values of the global color transform LUT are distributed into N layers, when N is calculated according to the equation T−1=2^(N); if T=5, we have N=2 layers (see FIG. 2); the highest layer n=1 (black dots on FIG. 2) contains (2^(n)+1)³=27 LUT output values, as the lowest layer n=2 (white dots on FIG. 2) contains (2^(n)+1)³−(2^(n−1)+1)³=125−27=98 LUT output values. In the general case of N layers, each layer n contains

$D_{n} = {\left( {2^{n} + 1} \right)^{3} - {\sum\limits_{i = 1}^{n - 1}\left( {2^{i} + 1} \right)^{3}}}$ color values. The first, highest layer n=1 requires 3.D.D₁=(2¹+1)³=81.D bits for the transmission of the output values of the LUT while the last, lowest layer n=N=log₂(T−1) requires

$D_{N} = {{3 \cdot D \cdot D_{N}} = {{3 \cdot D \cdot T^{3}} - {\sum\limits_{i = 1}^{{\log_{2}{({T - 1})}} - 1}{\left( {2^{i} + 1} \right)^{3}\mspace{14mu}{{bits}.}}}}}$

According to the example of this variant of the steps 2) and 10) of formation of a partial color transform LUT, the partial color transform LUT generating unit 14 chooses, from the N layers of the global color transform LUT that is stored in the color transform data storing unit 13, for each of (R_(g), G_(g), B_(g)) most frequent source color values, in order of decreasing frequency, the closest pair (R_(h), G_(h), B_(h), R′_(h), G′_(h), B′_(h)) of the global color transform LUT, searching for this closest color in the N layers starting from the highest layer, in order of decreasing layer, while searching already chosen entry colors are ignored, and ending with obtaining for each of those closest colors {(R_(h), G_(h), B_(h))\hε[1, Ñ_(Gk)], Ñ_(Gk)≦N_(Gk)} the (R′_(h), G′_(h), B′_(h)) destination color values are from the global color transform LUT, then forming a partial color transform LUT: {(R_(h), G_(h), B_(h), R′_(h), G′_(h), B′_(h))\hε[1, Ñ_(Gk)]} that is sent to the color transform LUT emission control unit 16. The number Ñ_(Gk) of colors is chosen with respect to the available transmission bandwidth as already explained.

Preferably, instead of transmitting a source color value or input within each pair of the partial color transform look up table, only the position of the source color values within its grid is transmitted; then each input-output pair contains this position as the input and the corresponding destination color value as previously; this variant can be advantageously used as soon as the source color values are regularly distributed within the source color space; one advantage of this variant is that the number of digits that are used to transmit each pair of the partial look up tables is lowered, then limiting the bandwidth requirements.

It will be understood that the present invention has been described purely by way of example, and modifications of detail can be made without departing from the scope of the invention. For example, it is possible to combine the method according to the invention with a method using the truncation of digits according to the prior art.

Each feature disclosed in the description and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination. Features may, where appropriate be implemented in hardware, software, or a combination of the two. Transmission link may, where applicable, be implemented as wireless transmission or wired, not necessarily direct or dedicated, transmissions.

Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

The present invention as claimed therefore includes variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art. While some of the specific embodiments may be described and claimed separately, it is understood that the various features of embodiments described and claimed herein may be used in combination. 

1. Method of transmission of at least one video sequence of images that is divided into a plurality of groups G₁, G₂, . . . , G_(k), . . . , G_(V) of successive images, comprising, for each group G_(k) of successive images, the following steps: transmitting, from an emitter, source color values that represent image(s) of said group G_(k) in a source color space, applying a color transform adapted to transform transmitted source color values into destination color values that represent image(s) of said group G_(k) into a destination color space, by using a resultant color transform look up table that is related to said group G_(k), wherein, for said color transformation: before the transmission of the source color values of the images of said each group G_(k), generating a partial color transform look up table {(R_(h),G_(h),B_(h),R′_(h),G′_(h),B′_(h))\h∈[1,Ñ_(Gk)]} related to said group G_(k) from a global color transform look up table representing said color transform, the size Ñ_(Gk) of said partial color transform look up table being smaller than the size (T³) of said global color transform look up table, transmitting said generated partial color transform look up table of said group G_(k), and reconstructing said resultant color transform look up table related to said group G_(k) by combining the transmitted partial color transform look up table of said group G_(k) with all the other partial color transform look up tables that have been previously transmitted and that are related to other groups of successive images, wherein the generation of the different partial color transform look up tables is performed such that none of these different partial color transform look up tables overlaps with another one; wherein, if said source color values are transmitted through an image channel having an image bandwidth BWIM and if a generated partial color transform look up table of said group G_(k) is transmitted through a color transform channel having a color transform bandwidth BWCT_(Gk), then the size Ñ_(Gk) of this partial color transform look up table is inferior or equal to: Q×M_(G(k−1))/(1+log₂T/D)×BWCT_(Gk)/BWIM, where Q is the number of pixels within each transmitted image, M_(G(k−1)) is the number of images in the previous group G_(k−1) of transmitted images, and D is the number of bits for a given color value.
 2. Method of transmission according to claim 1 wherein said generated partial color transform look up table of said group G_(k) is transmitted through a color transform channel, and in that the size Ñ_(Gk) of this partial color transform look up table is proportional to the bandwidth of said color transform channel.
 3. Method of transmission according to claim 1, wherein, during the transmission of the source color values of images of a group G_(k) through the image channel, a partial color transform LUT of a following group G(_(k+1)) is transmitted through a color transform channel.
 4. Method of transmission according to claim 1, wherein, before the transmission of the source color values of the images of said group G_(k): N_(Gk) more frequent source color values {(R_(g),G_(g),B_(g))\g∈[1,N_(Gk)]} of said group G_(k) are determined, then, to generate the partial color transform look up table {(R_(h),G_(h),B_(h),R′_(h),G′_(h),B′_(h))\h∈[1,Ñ_(Gk)]} related to said group G_(k), for each (R_(g), G_(g), B_(g)) of these frequent source color values {(R_(g),G_(g),B_(g))\g∈[1,N_(Gk)]}, a source color value or input (R_(h), G_(h), B_(h)) is extracted with its associated output (R′_(h), G′_(h), B′_(h)) from the source color values or inputs of the global color transform look up table which are the closest to said source color value (R_(g), G_(g), B_(g)) in the source color space.
 5. Method of transmission according to claim 4, wherein, input-output pairs of the global color transform look up table are distributed into N grids L₁, . . . , L_(n), . . . , L_(N) having increasing numbers X_(L1)<. . . <X_(Ln)<. . . <X_(LN) of input-output pairs from a highest grid L₁ down to a lowest grid L_(N) such that any source color value of the input-output pairs of a lower grid L_(n) is located, in the source color space, between adjacent source color values of its next higher grid L_(n−1): in order of decreasing frequency of the more frequent source color values {(R_(g),G_(g),B_(g))\gε[1,N_(Gk)]} of said group G_(k), for each (R_(g), G_(g), B_(g)) of these more frequent source color values, a source color value (R_(h), G_(h), B_(h)) or input which has not yet been previously extracted is extracted with its associated output (R′_(h), G′_(h), B′_(h)) from the highest grid L_(n) of the global color transform look up table, starting from the highest grid L₁ down to the lowest grid L_(N).
 6. Method of transmission according to claim 5 wherein, in each grid Ln, the inputs of the input-output pairs are regularly distributed in the source color space.
 7. Method of displaying at least one video sequence of images in a destination color space, wherein video sequence of images is transmitted by using a method of transmission according to claim
 1. 8. System of transmission of at least one video sequence of images wherein it is able to implement a method of transmission according to claim
 1. 